This invention generally relates to laser printers and more particularly, the present invention relates to laser printers having selectable printing resolutions.
Laser printers have become a de facto standard appliance in the art of printing. Laser printers print by depositing, using electrostatic charges, very finely manufactured toner on selected portions of print media such as paper then fusing the toner onto the paper using heat and pressure. Research in the laser printing technology resulted in the development of laser printers having resolutions in excess of 600 by 600 DPI (dots-per-inch) or even more. For such a printer, characters and marks on each page are defined by a grid of pixels, each pixel being 1/600th inch on each side.
Basic operations of a laser printer are discussed with reference to FIG. 1. FIG. 1 is a simplified schematic diagram of a laser printer 10. The laser printer 10 receives print data 12 from a host computer 14 connected to the laser printer 10. A processor 16 of the laser printer 10 formats the print data 12. That is, the processor 16 is adapted to process the print data 12 to produce pixel data representing a grid of pixels on a print medium such as paper. Often, each pixel is represented in the pixel data by one binary digit (a bit) with 0 assigned to one state (for example, off) and 1 assigned to another state (for example, on).
FIG. 2A illustrates a 12 pixel by 12 pixel sample grid 20 of pixel data. As illustrated, some pixels are in an on-state (for example, pixel 22) while other pixels are in the off-state (for example, pixel 24).
For each pixel of the pixel data, the processor 16 translates the pixel value to a pulse-width-modulation code, and sends the pulse-width-modulation code to a controller circuit 18 which controls the operations of various electro-optical-mechanical devices 19 of the printer 10. The electro-optical-mechanical devices 19 include, for example, a laser, a mirror, a drum, corona wires, rollers, lamps, a fuser, and various sensors. The sample pixel grid 20, when printed on paper, can appear as illustrated in FIG. 2B and forms a letter “A.” As illustrated, in the present example, to form the letter “A,” the print data 12 was formatted to pixel data including a grid of pixels having off-state (0) and on-state (1) values. Toner was applied to the print medium such as paper for the pixels represented by the on-states. Then, the toner is fused onto the paper using heat and pressure applied to the paper by a fuser.
For convenience, the pulse-width-modulation code is referred to as a pcode in this document. The processor 16 translates each pixel value into a pcode. The pcode is a set of two values often designated (j, p) where j is the justification value and p is the pulse width value which correlates to the width of pigmentation that can be laid down for a particular pixel. The pcode is used because, in most laser printers, the placement of the toner particles can have a finer adjustment even within a single pixel. Such fine adjustments are possible through the use of narrow laser beams, fine mirror controls, and very small toner particles.
For example, j can be set at 0, 1, 2, or 3 to indicate that the pigmentation for a particular pixel should be left justified, right justified, centered, or split within the pixel. Further, p can range within a predetermined range of numbers for adjustment of the pulse width value. Typically, p can range from 0 to 31 to provide for 32 different widths of the pigmentation within a pixel with 0 indicating no toner applied, 31 indicating toner applied to the entire width, and all numbers between 0 and 31 indicating toner applied to a corresponding portion of the pixel.
FIG. 2C illustrates pcodes for a portion 26 of the sample grid 20. For the sample grid 20, the off-pixels such as the off-pixel 24 have the pulse width value of 0 which correlates to no toner applied to these pixels. For these pixels, the justification value is not relevant since no toner is applied. The on-pixels such as the on-pixel 22 have the pulse width value of 31 which correlates to toner applied to the entire width of these pixels. Also for these pixels, the justification value is not relevant since the entire pixel is covered.
Other possible pcodes and their results as printed pixels are illustrated in FIGS. 3A and 3B. As illustrated, pixels 30, 32, 34, 36, 38, and 40 have pcodes, respectively, of (0, 0), (0, 15), (1, 15), (2, 7), (3, 15), and (0, 31). Pixel 30, having the pulse width value of 0, has no pigmentation within the pixel 30. For pixel 30 its justification value is irrelevant. Pixel 32 has pulse with of 15 (which is 16 counting from 0, 50 percent of 32 possible values) and justification code 0 (left justified). Accordingly, left 50 percent 33 of the pixel 32 is covered. Pixel 34 has pulse width of 15 and justification code 1 (right justified). Accordingly, right 50 percent 35 of the pixel 34 is covered. Pixel 36 has pulse width value of 7 (which is 8 counting from 0, 25 percent of 32 possible values) and justification code 2 (centered). Accordingly, center 25 percent 37 of the pixel 36 is covered. Pixel 38 has pulse width value of 15 and justification code 3 (split). Accordingly, 25 percent of left side and 25 percent of right side (collectively 39) of the pixel 38 is covered. Pixel 40, having the pulse width value of 31, the maximum value. Accordingly, the entire width of the pixel 40 is covered by the toner. For pixel 40 its justification value is irrelevant.
Generally, as illustrated in FIGS. 2A through 2C, each on-pixel is fully covered with pigmentation and each off-pixel is left completely uncovered. However, for certain characters or marks on the print medium, such technique imposes limitations, lead to undesirable results, or both.
For example, it would be desirable to adjust or set the pcodes of certain pixels to vary print quality, to produce desired effects on the print medium, alleviate or eliminate unwanted results, or a combination of these.
One approach to vary print quality (for example to produce a draft quality document while reducing toner usage) is to use a single, predetermined pcode (rather than the translated pcode) for selected pixels. However, this approach is limited to using the same predetermined pcode irrespective of the context in which the pixel was selected. Thus, the predetermined pcode may not be the optimum, or even appropriate, pcode for the selected pixel. Further, such approach does not address certain undesirable effects such as toner explosion.
Toner explosion occurs during the fusing process. When the fuser applies heat and pressure to paper and toner on the paper, the toner melts and fuses with the fibers of the paper. During the fusing process, the heat and the pressure generate gas and water vapor within the paper. Normally, such gas and water vapor leave the paper without interfering with the melting toner; however, certain patterns of toner prevent efficient ventilation of the gas and water vapor leading to scattering of toner on the page resulting in a defective image. The current techniques are limited in their ability to eliminate such conditions.
Accordingly, there remains a need for an improved method and apparatus to alleviate or eliminate the shortcoming of the current generation of printers.